This invention relates to a method of making cationic dialdehyde starch compositions which are useful as wet- and dry-strength additives for paper and paper products.
It is common practice in the manufacture of paper to apply expensive specialty chemicals to the papermaking process, e.g., to the fiber slurry or "wet end," in liquid or solution form. It is obvious that preparation of such chemical solutions, whether done at the paper mill or at a supplier's manufacturing plant, is most economical when made at as high a concentration as possible without adversely affecting fluidity of the final product. It is particularly desirable to make high concentration of those chemicals, such as cationic dialdehyde starch, which require heat input during the dispersion process, as the energy cost per unit of cationic dialdehyde starch is, thereby, decreased. Also, shipping and storage costs are less per unit as the solids concentrations increase. In the specific case of cationic dialdehyde starch, which is synthesized in situ in the dispersion process by reaction of dialdehyde starch (DAS) and betaine hydrazide hydrochloride, further significant economy is effected during preparation when the quantities of the relatively more expensive betaine hydrazide HCl are minimized while maintaining the effectiveness of the cationic dialdehyde starch to provide wet and dry strength to paper.
Cationic dialdehyde starch products for use as dry- and wet-strength agents in papermaking have been prepared by simultaneously derivatizing and dispersing DAS in dilute (3-5%) aqueous media. Thus, Mehltretter (U.S. Pat. No. 3,251,826) formed cationic starch hydrazones by introducing DAS to water (2.9% DAS) followed by 5% (DAS basis) betaine hydrazide hydrochloride dissolved in a few milliliters of acidified water. The mixture was heated (90.degree. C.) and stirred for 0.5 hour to complete dispersion.
Previous investigations [Hamerstrand et al., Tappi 46(7): 400 (1963)] using DAS and amounts of cationic hydrazide to give from 0.02 to 0.14 degree of substitution achieved optimum wet-strength development at 0.05 degree of substitution (5% betaine hydrazide hydrochloride, DAS basis).
Prior research utilizing cationic DAS dispersions for wet strength in paper describes only 3-5% DAS concentrations. These low concentrations would cause considerable difficulties in readying large (1000 gallons or more) batch quantities in a paper mill. On the other hand, if such low-concentration dispersions were manufactured it would be impractical (economically) to ship them to distant locations.
The prior art teaches and emphasizes that reaction time between DAS (3% concentration at about 90.degree. C.) and betaine hydrazide hydrochloride must not appreciably exceed 0.5 hour because further heating progressively degrades DAS which then becomes less effective as a wet-strength agent. We were surprised, therefore, when we found a method of making stable, nongelling, high-concentration aqueous cationic dialdehyde starch compositions useful for imparting wet and dry strength to paper and paper products comprising reacting dialdehyde starch in aqueous dispersions at concentrations of about 15% by weight with betaine hydrazide hydrochloride in amounts equal to from 3-5% of the dry weight of DAS at from 90.degree. to 95.degree. C. for a time sufficient (i.e., from 2.3 to 3.6 hours depending on the amount of betaine hydrazide hydrochloride) to form aqueous dispersions having a pH of from 2.5 to 3.5 which remain fluid when cooled to 25.degree. C.
The product of the invention has all of the above-mentioned advantages of a high-concentration dispersion plus the added advantage of being more effective than prior art cationic DAS dispersions in that a composition prepared according to the invention containing 3% betaine hydrazide hydrochloride gave wet strength essentially equivalent to that given by a prior art composition containing 5% betaine hydrazide hydrochloride, all other parameters being equal.